The very thin stratum corneum (SC), 10–30 µm or 1/10 the thickness of a piece of paper, is the skin’s outer and most exposed layer, and the permeability barrier between the internal milieu and hostile environmental assault. Because it is exposed externally, the skin provides protection and acts as a barrier to the outside environment in order to maintain internal homeostasis. Although the SC has many functions, its main ability is to serve as a protective barrier that prevents excess fluid and electrolyte loss, allowing life to exist in a fragile terrestrial environment.
A wall of protection
The SC accounts for up to 75% of the epidermis, making it a virtual wall of protein protection for the cellular and inner-cellular inhabitants. “Stratum Corneum Defense Functions: An Integrated View,” written by Peter M. Elias, MD, states, “The permeability barrier is mediated by the organization of the extra cellular lipids of the SC into a series of parallel membrane structures, and its distinctive composition.”1
This fascinating upper layer of the epidermis is composed of fibrous protein-enriched corneocytes and an essential lipid-enriched intercellular matrix. During corneocyte formation, the plasma membrane of viable cells is replaced by a ceramide monolayer, covalently bound to the underlying cornified envelope. This lipid envelope functions as a template for accumulating extra cellular lipid layers.